The invention relates to a filter element for use in a unit used to filter or purify liquid. In particular, the invention is directed to a filter element having two types of layers of windings over a central support core, and to a method of purifying liquids with such an element.
In the prior art one type of apparatus for filtering liquids typically includes a perforated cylindrical support core and windings of strands of material around the support core, such as disclosed in Goldman U.S. Pat. No. 1,751,000. These filters are adapted to remove undissolved particles from liquid as the liquid is passed through the filters in a direction from the outside, through the wound material, and into the support core. The undissolved particulate matter is trapped by the wound material.
Other filter elements in the prior art disclose a number of layers of material wound around a central support core, wherein the layers vary in density. As disclosed in Snow U.S. Pat. No. 3,680,709, the density decreases in a radial direction away from the support core because the windings are tighter close to the support core and are gradually reduced toward the outside.
It has been found that such prior art filter elements having a plurality of layers, wherein outermost layers are less dense than the innermost layers, present difficulties resulting from fouling tendencies, backwashing limitations, and inadequate flow distribution. These disadvantages are particularly noted when the wound filter elements of the prior art are used to support a precoat of particles in the size range of about 60 to 400 mesh. The precoat particles typically are particles of cation and anion exchange resins. These precoated filter elements are used to purify water by reducing dissolved and undissolved impurity concenrations from levels of approximately 50 parts per billion to less than 10 parts per billion.
Filter elements of the prior art having a graduated density of windings decreasing in a direction away from the central support core are easily blinded by particulate matter which infiltrates towards the center of the wound layer or layers, as liquid is passed through the filter element in a service cycle direction. Because of close spacing of windings near the support core, it is difficult to achieve velocity flows of liquid in a backwash direction through the filter element sufficient to dislodge and remove particulate matter which is trapped within the filter element windings.